Landscape Ecology of Coral Reefs Connections of Coral Reefs to Mangrove and Seagrass Systems

Coral reefs are typically found in close proximity to other coastal ecosystems, particularly seagrass beds and mangrove forests. These different ecosystems are often connected to reefs via the movement of animals and nutrients across their boundaries. For example, carnivorous grunts (Haemulidae) forage in seagrass beds at night but school around large coral heads on reefs during the day as a refuge from predation. Coral heads that harbor fish schools receive nutrient supplements from fish excretion, grow up to 23% faster, and have more nitrogen and zooxanthellae per unit area than do corals without resident fishes. Thus, fishes that have no direct trophic link with corals collect nutrients from other ecosystems (seagrass beds) and concentrate these near their host coral. This facilitates coral growth, enhancing the coral's value as a refuge for these fishes and for other reef organisms.

Mangroves and seagrass beds also serve as nursery grounds and provide refuge from predators and an abundance of food for many juvenile fishes that are typically found on coral reefs as adults. Grunts (Haemulidae), snappers (Lutjanidae), barracuda (Sphyraena barracuda), and some parrotfishes (Scaridae) are particularly dependent on the presence of nearby mangroves. In Belize, reefs closely associated with mangroves have up to 26 times more biomass of some species of fish than reefs not associated with mangroves. A common species on these reefs, the bluestriped grunt (Haemulon sciurus), typically goes through an ontogenetic change in habitat use as it migrates from seagrass beds to mangroves to patch reefs to the forereef as it ages (Figure 7). In areas where mangroves are absent, bluestriped grunts move from sea-grass beds directly to patch reefs and are typically smaller than grunts that inhabit patch reefs with nearby mangroves. Thus, mangroves provide important habitat

Mangroves present

Mangroves present

Mangroves Patch reef Shallow forereef Montastraea reef

Seagrass beds

Mangroves Patch reef Shallow forereef Montastraea reef

Seagrass beds

Mangroves absent

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Patch reef Shallow forereef Montastraea reef

Seagrass beds

Patch reef Shallow forereef Montastraea reef

Seagrass beds

Figure 7 Schematic illustrating the connection between mangroves and coral reefs. Ecosystem connectivity is stylized for Haemulon sciurus (gray and black fish) and Scarus guacamaia (orange and green fish) although other parrotfish (scarid), grunt (haemulid), and snapper (lutjanid) species also exhibited similar ontogenetic shifts in habitat use. H. sciurus show a substantial shift in size frequency from seagrass (A) to mangroves at approximately 6 cm. On reaching a given size in seagrass beds, juvenile fish then move to mangroves (B) which serve as an intermediate nursery habitat before migrating to patch reefs (C). If mangroves are not present, H. sciurus move directly from seagrass to patch reefs, appearing on patch reefs (G) at a smaller size and at lower density (260 ha-1 compared to 3925 ha-1 in mangrove-rich systems). In the presence of mangroves, the biomass of H. sciurus is significantly enhanced on patch reefs, shallow forereefs, and Montastraea reefs (C, D, E). S. guacamaia (F) has a functional dependency on mangroves and is not seen where mangroves are absent. Illustration describes findings from Mumby PJ, Edwards AJ, Arias-Gonzalez JE, etal. (2004) Mangroves enhance the biomass of coral reef fish communities in the Caribbean. Nature 427: 533-536. Schematic and description courtesy of Peter J. Mumby.

where juvenile grunts feed and increase in size before moving to patch reefs which may subsequently decrease the threat of predation once they move to these reefs. Further, the rainbow parrotfish (Scarus guacamaia), the largest herbivorous fish in the Caribbean, is functionally dependent on mangroves for shelter; juveniles of this species live primarily in mangroves, and the species goes locally extinct on reefs when nearby mangroves are removed (Figure 7). Interestingly, density of fishes that have no direct link to mangroves at any stage of their life history can still be influenced by the proximity of mangroves, probably via interactions with mangrove-dependent fishes. Thus, the composition of the fish community on reefs is greatly influenced by the proximity of mangroves, and the rapid removal of mangrove forests from coastlines worldwide will certainly have drastic negative impacts on the ecology coral reefs.

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